1. Field of the Invention
The present invention relates to a disk device and a disk reproducing method, and more particularly, to a disk apparatus and a disk reproducing method concerning limitation on coefficient control of a waveform equalizing circuit of a reproducing system.
2. Description of the Related Art
Recently, an optical disk recording and reproducing apparatus such as a DVD (Digital Versatile Disc) has been widely known, and research and development of a variety of schemes have been made and a plenty of products have been commercially available.
In particular, in recent years, in the field of optical recording and reproduction as well, recording with high density has been underway, and the recording density in a linear direction has been significantly increased. Since a laser wavelength is reduced, and a degree of aperture of a lens is increased, deterioration of a reproducing signal quality due to a tangential tilt becomes significant. Therefore, in order to solve these problems, development of a PRML (Partial Response and Maximum Likelihood) signal processing scheme is actively made.
The PRML signal processing system achieves a reproducing system which does not require a high bandwidth component by applying a PR scheme for intentionally adding waveform interference in a reproducing system in which an amplitude of the high bandwidth component deteriorates and a signal to noise ratio increases due to an increase of the recording density in a linear recording direction. In addition, this scheme improves a quality of reproducing data by using a likelihood decoding technique for estimating the most probable sequence by probability calculation considering the waveform interference. In this PRML scheme, a plenty of PR classes are produced from the shape of given waveform interference. In particular, in an optical disk apparatus, there is used a PRML scheme which corresponds to a response such as PR(1, 1), PR(1, 1, 1), PR(1, 1, 1, 1), PR(1, 2, 2, 1), PR(3, 4, 4, 3), or PR(, 2, 2, 2, 1).
In patent document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2001-195830), a PRML signal processing scheme in the above-described optical disk apparatus is disclosed in detail.
In the meantime, in the case of utilizing the PRML signal processing scheme, it is necessary to carry out a waveform equalizing process called PR equalization in order to eliminate a waveform distortion of a reproducing waveform and make it close to an equalization characteristic of a target PR class. In general, an adaptive equalizer having a learning function is used for this waveform equalizing process.
In non-patent document 1 (Journal of the Institute of Electronics, Information Communication Engineers, Vol. 81. No. 5, pp. 497 to 505, May, 1998), adaptive equalizers are described in detail. Among them, in particular, an adaptive equalizer using an LMS (Least Mean Squares) algorithm for updating a tap coefficient so as to minimize a square average value of an equalization error is practically used for an optical apparatus as a scheme which can be achieved with a simple configuration.
However, in the above-described prior, it cannot be considered that adequate countermeasure is taken in view of divergence of a tap coefficient and prevention of error convergence in the optical disk apparatus. That is, the divergence of an adaptive leaning or incorrect convergence that the adaptive leaning incorrectly converges at a point which is different from a target equalization characteristic occurs for the reasons stated below.
First, in an optical disk recorded with high density, in response to a channel rate of a reproducing signal, a high bandwidth component in a bandwidth of the reproducing signal is greatly eliminated with an MTF (Mutual Transfer Function) characteristic. Thus, pseudo convergence may occur at a point close to the target PR class characteristic while this high bandwidth portion is forcibly emphasized. In the view of a circuit, since the value of each coefficient cannot be changed infinitely, a limitter is applied to any coefficient anywhere. Then, convergence occurs in a state in which an equalization error is reasonable or divergence occurs, making it impossible to return to an intrinsic characteristic. In actuality, such a state exists in plurality.
Secondly, in the case where a nonlinear factor or a disturbance factor such as a tangential tilt, asymmetry, a phase error, or defect has been temporarily inputted greater than expected, a learning operation advances so as to correct this factor, and a coefficient learns in a direction in which such a factor is eliminated. As a result, there is a danger that pseudo convergence occurs in a characteristic which is not an intrinsic target characteristic, and an original state cannot be returned again. In addition, it is not unusual that the weighting of a tap which should be essentially central is reduced, a characteristic is produced with only another tap, or convergence occurs around another tap.
Thirdly, in the case where the PRML signal processing is composed of a plurality of loops, another loop such as gain control, offset/asymmetry control, or phase control exists at the outside of a loop of an adaptive equalizer. Thus, there is a case where an adaptive leaning operation becomes unstable due to an operating collision.
As has been described above, in an adaptive equalizer, a problem with divergence of leaning or incorrect convergence can occur due to a variety of factors. As an approach for solving such a problem, there is conventionally known a method for limiting a degree of freedom of a tap coefficient. For example, there is used a method for fixing a central tap coefficient to be present to a stable value obtained by experimentally calculating the central tap coefficient, or alternatively, disabling learning of some tap coefficients. Further, there is proposed a method for carrying out leaning while maintaining symmetry of tap coefficients in order to retain a linear phase characteristic.
In patent document 1, there is disclosed an example for preventing divergence while providing a degree of freedom by providing a margin to limitation on updating between symmetrical tap coefficients in the optical disk apparatus.
Further, patent document 2 (U.S. Pat. No. US6,618,740B1) exists.
However, in a prior art of patent document 1, if limitation is thus strictly applied to a leaning operation, a degree of freedom corresponding to a target characteristic is also lost. Therefore, there is a possibility that an equalization error quantity at a convergence point cannot be sufficiently reduced. In addition, as in a prior art of patent document 2, an attempt has been made to increase a degree of freedom while providing a limitation on updating between symmetrical tap coefficients. However, this attempt is not always considered to have a high degree of freedom because limit values between all the tap coefficients are identical to each other. Further, there has been a problem that a sufficient result can be obtained in the case where an attempt has been made to correct a specified amount of a tangential tilt or asymmetry etc. by an adaptive equalizer. In addition, in the case of an optical disk apparatus, there exist a plurality of standards for optical disks with different recording densities. In the case where reproduction is carried out by a single apparatus in accordance with these standards, there has been a problem that the specified amount must be changed according to the characteristics of their respective reproducing disks.